1. Field of the Invention
The invention relates to optical filters, their production and their uses. In particular, present invention provides neutral density filters (ND filters) which have heat-resistant, mechanically stable absorption layers or filter layers. A lens or objective equipped with the filter of the invention is also provided by the invention.
2. Description of Related Art
A neutral density filter (German: “Graufilter” or “Neutraldichtefilter”), reduces the quantity of light coming through a lens. This leaves the colour character of the light unchanged. Absorption filters can consist of an absorbing glass, an absorbing polymer, thin polymer layers on glass or of coated glasses.
Bulk-coloured ND filters have very different colorant concentrations. Thus, there are firstly very light-grey variants having a low colorant concentration and secondly also ND filters which appear black to the naked eye, e.g. the SCHOTT glass NG which at a thickness of 1 mm has a transmission of less than 10−3.
However, absorbing glasses are generally very thick as components and therefore cannot be used everywhere. Filters composed of absorbing polymers are likewise usually excessively large components and the thermal stability which is sometimes necessary and the optical properties are not satisfactory. In addition, the lifetime of polymers is insufficient.
Filters based on thin polymer layers are likewise not thermally stable enough for particular applications. This is usually due to the unsatisfactory thermal stability of the polymeric materials. Depending on the polymer, the material is stable only up to 120° C. For example, U.S. Pat. No. 4,738,798 A discloses a coating comprising nanoparticles which are incorporated into a polymer matrix which is not suitable for high temperatures.
Furthermore, absorbing colorants which are also organic materials and also have a reduced thermal stability are often used in polymeric filters. In addition, polymeric materials often have an unsatisfactory life or the optical and mechanical properties change during the life of the materials.
Filters based on coated glasses are typically based on vapour-deposited metal layers (for example an Inconel coating). Manufacture is very expensive and not suitable for large-area filters.
DE 10 2009 036 134 A1 discloses coatings comprising nanoparticles which are so large that they lead to light scattering. However, scattering is undesirable in optical filters.
US 2010/0246009 A1 discloses nanoparticles in a matrix which produce a particular colour or protect against UV rays.
It is therefore an object of the present invention to provide optical neutral density filters which are at the same time thermally stable, mechanically strong and can be produced inexpensively in small sizes.